1. Field of the Invention
The present invention relates generally to an operation mode monitor for a microcomputer, and more specifically to a monitor for checking whether or not a predetermined operation mode has correctly been selected in a microcomputer in response to an operation mode setting signal generated from an external circuit whenever the microcomputer is initialized.
2. Description of the Prior Art
Recently, a microcomputer is widely used for an automotive vehicle to electronically control an engine, an air conditioner, etc. The microcomputer encapsuled within a chip is usually provided with a microprocessing unit (MPU) for executing various calculations or operations in accordance with control programs, a random-access memory (RAM) for temporarily storing various data, a read-only memory (ROM) for storing the control programs, and input and output ports. A predetermined program control for an automotive vehicle can be implemented by connecting various sensors and actuators to the input/output ports of the microcomputer.
However, the memory capacity of the ROM housed within a microcomputer chip is usually as small as 2K bite or byte (one bite means a group of binary digits considered as one unit or one word). Therefore, this memory capacity is not sufficient for a certain control program. In order to increase the memory capacity, some ROMs are externally connected to the microcomputer. In this case, operation mode in which only the internally-housed ROM is used is called single-chip mode; operation mode in which externally-connected ROMs are used is called extended mode. To use such a microcomputer, the user must previously select any one of a plurality operation modes and the selected operation mode is initially set in the microcomputer, whenever the microcomputer is initialized, in response to a mode setting signal generated from an external circuit.
To give an example of such a microcomputer in which operation modes can previously be selected, there exists a microcomputer of HITACHI HD 6801. In this model, one of three operation modes of single chip mode, extended non-multiplex mode and extended multiplex mode can be selected by inputting a predetermined external signal corresponding thereto through three specific input port terminals.
However, in the case where such a microcomputer as described above is mounted on an automotive vehicle body, since the operation mode setting signal is susceptible to intense noise generated thereabouts, there exists a problem in that an erroneous operation mode different from the one corresponding to the predetermined operation mode setting signal may be selected. In case an erroneous operation mode has been selected in the microcomputer, since sensors or actuators are all arranged for the microcomputer so as to operate correctly only when a predetermined single operation mode has correctly been selected, it is impossible to execute normal calculations, operations or control.
In this connection, in case such an erroneous operation mode has been selected, although it is possible to select a correct operation mode readily by resetting the microcomputer again, since it is relatively difficult for the user to know that the microcomputer is set to an abnormal operation mode, there exists another problem in that the user cannot take an appropriate action promptly.
A more detailed description of the prior-art microcomputer in which any one of operation modes can be selected will be made with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.